A variety of different types of zeolites is known in the art, including natural and synthetic zeolites. Research has opened up a spectrum of new opportunities in the field of molecular shape selective catalysis, where the intracrystalline space accessible to molecules has dimensions near those of the molecules themselves. This field is discussed in an article titled "Molecular Shape Selective Catalysis", P. B. Weisz, New Horizons in Catalysis, Part A, 1980. For example, it is possible to catalyze the dehydration of n-butanol over a Linde 5 .ANG. zeolite without reacting isobutanol which may be present. Such research has led to the concept of molecular engineering.
Early work was very limited by the choice of zeolites. This limitation lead to the discovery of methods for zeolite synthesis, using large organic cations as templates in place of the traditional all-inorganic ionic species. This research opened the way to the synthesis of many new zeolites. Now a number of industrial processes, including selectoforming, M-forming, dewaxing, xylene isomerization, ethyl benzene production, toluene disproportionation and methanol-to-gasoline are based on shape selective zeolites. Since the early demonstrations of product selectivity, many more cases have been observed and many reviewed and reported by Csicsery and Derouane. See S. M. Csicsery, "Zeolite Chemistry and Catalysis", ACS Monograph 171, J. A. Rabo, Ed., American Chemical Society, Washington, D.C.(1976), and E. G. Derouane, "Diffusional Limitations and Shape Selective Catalysis in Zeolites", from Intercalation Chemistry, M. S. Whittinham, A. J. Jacobson, Eds., Academic Press, New York.
There is a review of molecular sieve zeolites used in catalysis titled "Molecular Sieve Catalysis", J. W. Ward, Applied Ind. Catal., Vol. 3, 1984. Though zeolites have been known for a long time, the major stimulus in molecular sieve science came with the first synthesis of A zeolite by Milton, described in U.S. Pat. No. 2,882,243 (1959).
The natural crystalline aluminosilicate zeolites can be represented by the empirical formula: EQU M.sub.2 /.sub.n O.multidot.Al.sub.2 O.sub.3 .multidot.xSiO.sub.2 .multidot.yH.sub.2 O
The synthetic X and Y type zeolites have framework structures similar to that of the natural mineral faujasite although they are distinct species. The unit cells are cubic with a cell dimension of nearly 25 .ANG.. Each unit cell contains 192 SiO.sub.4 and AlO.sub.4 tetrahedra that are linked through shared oxygen atoms. Ibid., p.274.
In the Y zeolites the three-dimensional framework comprising a tetrahedral arrangement of connected truncated octahedral provides giant supercages approximately 13 .ANG. in diameter with eight supercages per unit cell. The supercages are interconnected by twelve-membered rings of about 8 .ANG. in diameter. Many different chemical species can be absorbed by this large-pore system. Ibid., p. 275.
Various zeolites have characteristic structures which favor certain types of reactions. For instance, mordenite is one of the most silica-rich zeolite minerals, having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 10. The structure consists of chains of tetrahedra cross-linked by the sharing of oxygen atoms. Mordenite has high thermal stability, probably due to the presence of the large number of five-membered rings that are energetically favored. The dehydrated structure has a two-dimensional channel system accessible to small molecules, but not to typical hydrocarbon molecules. Ibid., pp. 275-6.
Erionite is probably the smallest pore zeolite used commercially.
A number of zeolites and molecular sieves have been synthesized that have SiO.sub.2 -Al.sub.2 O.sub.3 ratios greater than 10 or are essentially pure silicas. Examples of those which have found commercial utility because of their shape selective properties are ZSM-5 and ZSM-11. Some of these zeolites are aluminum-free silicalites which have no ion-exchange properties and should properly be regarded as molecular sieves.
Zeolite molecular sieves can be modified by treatment by cation exchange, thermal or hydrothermal treatment and chemical modification. Most catalytic preparations involve an ammonium ion exchange, typically by refluxing the zeolite with at least a five-fold excess of aqueous ammonium salt.
Divalent cation exchange with elements such as calcium and magnesium is considered rather difficult according to Ward.
Rare earth ion exchange zeolites have played an important role in zeolite catalysis, particularly in fluid cracking catalysts and require multiple batch exchanges at elevated temperatures with excess solutions. Ibid., pp. 288-289.
Zeolites having higher silica/alumina ratios are more stable and, therefore, more suitable for treatment. Careful acid treatment can result in up to 75% of the alkali metal ions being replaced before structural collapse occurs. Ibid., p. 290.
The thermal or hydrothermal treatment of zeolites is also known. Thermal treatment of synthesized X and Y zeolites has no structural effects on the zeolite until the decomposition temperature of about 800.degree. C. is reached. It is possible to exchange and reexchange ions. For instance, it is possible to exchange with ammonium ions, calcine and exchange with rare earth. Ibid., p. 292.
Zeolites lose physically bound water to form an endotherm on heating to about 150.degree. C. and they form exotherms around 800.degree. C. which represent structural collapse of the zeolite. The hydroxyl groups are believed to be in different parts of the structure, some in supercages and some inaccessible to most absorbing molecules.
Zeolites can be modified to remove alumina by treatment with chelates such as acetylacetone and ethylenediamine tetraacetic acid. Aluminum atoms can be replaced with silicon tetrachloride or treatment with ammonium fluorosilicate. Ibid., p. 298. The authors of this article did not appear to contemplate the possibility of impregnation of certain small pore zeolites with alkali metal halides.
An article titled "Catalysis on Faujasite Zeolites", R. Rudham et al., Specialist Periodical Report, Chemical Society, 1977, offers an additional review of catalytic and structural properties of X and Y zeolites. They are members of the isostructural group of faujasite zeolites, which also include the rare mineral faujasite and a number of synthetic zeolites in addition to X and Y. The differences can be structurally represented by the following:
______________________________________ Faujasite (Na.sub.2,K.sub.2,Mg,Ca).sub.29.5 [(AlO.sub.2).sub.59 (SiO.sub.2 ).sub.133 ]235H.sub.2 O Zeolite X Na.sub.86 [(AlO.sub.2).sub.86 (SiO.sub.2).sub.106 ]264H.sub.2 O Zeolite Y Na.sub.56 [(A1O.sub.2).sub.56 (SiO.sub.2).sub.136 ]250H.sub.2 ______________________________________ O
The term aluminum-deficient zeolite can be applied to a faujasite zeolite from which, for example, both framework aluminum and cations have been extracted by treatment with ethylenediaminetetra-acetic (EDTA).
A number of commercial applications of synthetic zeolites are discussed in "Synthetic Zeolites in Commercial Applications", R. G. Muller et al, SRI PEP Review v. 81-3-3(1982). Due to the unique structure of zeolites and to the knowledge available today regarding properties and manufacturing processes, many uses have been discovered for zeolites in adsorbent and catalytic applications. Some of the reactions for which synthetico zeolites have been shown to be active catalysts include xylene isomerization, naphtha isomerization, light olefin oligomerization, toluene dealkylation, benzene hydrogenation, olefin and fat hydrogenation, methanation, dehydrogenation of ethylbenzene, dehydrohalogenation, desulfurization and propylene carbonylation.
A good means of familiarization with the relationship between molecular shapes, structures of zeolites and selectivity for certain catalysis is available in an article titled "Industrial Application of Shape Selective Catalysis", N.Y. Chen et al., Catal. Rev.-Sci. Eng., 28 185 (1986).
The zeolites of interest to shape-selective catalysis may be divided into three major groups according to their pore/channel systems. The first group includes 8-membered oxygen ring systems such as, for example, zeolite alpha, ZK-4, ZK-21, ZK-22 and several other less common natural zeolites.
The second group includes 10-membered oxygen ring systems such as, for example, ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-48 and laumontite, which has a puckered 10-membered oxygen ring. The rest are considered medium pore zeolites and are usually synthetic in origin; they are sometimes known as pentasils. They have a predominance of silicon.
The third group of zeolites is those having dual pore systems which are interconnecting channels of either 12- and 8-membered oxygen ring openings or 10- and 8-membered oxygen ring openings.
An article by D. S. Shihabi et al., J. Catal., 93, 471 (1985) showed that binding of high-silica ZSM-5 with alumina enhances the catalytic activity of the catalyst for numerous reactions. In an article by C. D. Chang et al., J. Am. Chem. Soc., 106, 8143 (1984), there is reported aluminum insertion into high-silica zeolite frameworks by reaction with aluminum halides.
There are a number of different methods of achieving molecular shape selectivity. Shape selectivity can be accomplished either through reactant selectivity or product selectivity. It is believed Columbic field effects also play a part. Another phenomenon which has been observed to contribute is configurational diffusion which occurs in situations where structural dimensions of the catalyst approach those of molecules; even subtle changes in dimensions of molecules can result in large changes in diffusivity. Chen et al., Catal. Rev.-Sci. Eng., supra, p. 198.
Another type of selectivity which has been observed is spatiospecificity or restricted transition state, where both the reactant molecule and the product molecule are small enough to diffuse through channels, but the reaction intermediates are larger than either the reactants or the products and are spatially constrained. This is one of the most important properties of ZSM-5. Some zeolites, such as ZSM-5, ferrierite, cliniptilolite, offretite and mordenite have intersecting channels of differing channel size. Ibid., p.198. It is noted that these zeolites are preferred in the instant invention.
Reactants which are of interest in shape selective catalysis include hydrocarbons, paraffins, olefins, naphthenes and aromatics.
In U.S. Pat. No. 4,214,307 to C. D.Chang el al.(Jul. 22, 1980), it is shown that hydration of C.sub.2 to C.sub.4 olefins to alcohols can be carried out over ZSM-5 at below about 240.degree. C. and 10 to 20 atmospheres of pressure without forming ethers or other hydrocarbons, however, above 240.degree. C. propene and butenes undergo other olefinic reactions, forming higher molecular weight hydrocarbon products.
U.S. Pat. No. 4,760,200 provides a good background for the production of alkylene glycols. The process claimed therein is for the selective production of monoalkylene glycol which comprises reacting a vicinal alkylene oxide in liquid phase in the presence of a water-soluble metalate anion catalyst, wherein the aqueous medium also contains a water miscible ethylene glycol ether co-solvent.
In current typical alkoxylation processes, aqueous potassium hydroxide solution is commonly used as a catalyst and is reacted with alcohols to form initiators. The excess potassium hydroxide and potassium alkoxide are neutralized with a suitable organic acid such as oxalic or a mineral acid such as sulfuric acid and/or Magnesol.RTM. magnesium silicate. The resulting salts or filter cake must then be filtered from the reaction mixture. This magnesium silicate causes both product loss and presents a waste disposal problem. Therefore, it would be very desirable to develop a solid base catalyst which could be as effective as aqueous KOH for alkoxylation.
From a review of the art available there are no descriptions of alkali metal, alkaline earth metals and alkali metal halides on certain small pore and/or dual pore zeolites, aluminas, silica/aluminas and zeolite aluminas.
Triethylene glycol is a valuable gas treating compound. It is also a useful chemical intermediate. For example, it has been used in the manufacture of triethyleneglycoldiamine. It has excellent solvent properties and is useful in reactions which require high temperatures. The triethylene glycol is typically obtained as a by-product of the reaction between ethylene oxide and water to prepare ethylene glycol. It would be preferred to prepare triethylene glycol directly such as by the reaction of diethylene glycol with one mole ethylene oxide. However, with conventional alkoxylation catalysts, the reaction is not selective and a wide distribution of products including "heavies" are formed. This cuts down on the triethylene glycol yield and results in the formation of non-useful heavy materials. Using the process of the instant invention it is possible to produce a narrow range of glycols with minimal production of bottoms.